https://superuser.com/questions/72512/best-irc-client-for-windows was closed as a duplicate in 2009, but less then 10 minutes ago was protected. Is there a reason/rational for this? I assumed closed questions could not be posted on anyways, so I am not understanding why it needs protection
As @Nifle said, you could protect a question so it won't attract spam (or otherwise bad answers) when it's reopened.
Since the question about the best IRC client for Windows … cough doesn't seem to be around anymore, and there never was a good chance of it being reopened anyway, I don't think it would have been necessary to protect it in the first place.
Good example for questions that need protecting are those that get immensely popular over a few hours or days and, due to that, get more non-answers than necessary. Don't forget that the Community user auto-protects posts as soon as three answers by new users are deleted. Those popular questions are also the ones that might go through several close–reopen–close cycles.